In a typical digital wireless radio telephone system, multiple mobile stations (or subscribers) and a respective base station communicate across TDMA channels. Data is transmitted in frames, each of which comprises a number of time slots. The multiple mobile stations share the same transmission and reception frequencies, but are assigned separate time slots within those frequencies. In normal operation, the mobile stations transmit bursts of data at certain times allowing for propagation delays so that the bursts are received at the base station in their assigned time slot. Otherwise, transmissions from different mobile stations would collide or overlap, resulting in mutual interferences in the reception at the base station. Typically, a guard time is inserted in the time slots to help prevent the bursts from colliding. Such collisions occur if the time slots are exposed to variations in the propagation delay between a mobile station and the base station.
FIG. 1 illustrates a TDMA frame structure based on the IS-54 transmission standard. Each frame 100 has a duration of 40 msec and includes six time slots 102, each having a duration of 6.67 msec. Each frame 100 is comprised of 1944 bits, or 972 symbols, where each symbol is comprised of two bits. Each time slot 102 contains a 28-bit sequence as a preamble 104 which is used for synchronization. The remainder of the time slot 106 contains digitized voice data (DATA) along with six bits necessary for a guard time (G) that is used to avoid burst collisions, a ramp time (R) for the transmitter in the mobile station, in-band signaling through a slow access control channel (SACCH), synchronization and training (SYNC), and digital verification color code (DVCC).
In order for a mobile station to transmit, the mobile station must identify and locate the beginning of an assigned time slot. The identification of the start of the time slot is referred to as synchronization. The propagation delay incurred in transmitting data between two communicating stations effects the identification of the mobile station's time slot. The propagation delay varies as the distance between a mobile station and the base station varies.
In some systems, the guard time is selected to be small for spectral efficiency. A time advance is used to compensate for the propagation delay. The time advance indicates the time at which the mobile station advances transmission of the message in order for a receiving station to receive a burst at the designated time slot. Initially, the time advance is determined by the base station and provided to the mobile station through signaling. The time advance is based on a coarse approximation of the distance between the mobile unit and the base station. This approximation is then refined during subsequent transmissions. The base station calculates a time offset that is used to adjust the time advance. The base station determines the time offset from the time that it transmits and receives bursts to and from the transmitting station. Each newly calculated time offset is then provided to the transmitting station on the SACCH.
This type of system suffers from the additional overhead used to determine the time advance and the time offset and to communicate these values between the two communicating stations. This is attributable to the coarse measurement that is used for determining the distance between the two communicating stations. This distance effects the propagating delay thereby requiring the time offset to be continuously updated and transmitted to the transmitting station. Accordingly, there exists a need for a synchronization mechanism that can overcome this shortcoming.